Process and device for reducing pollutants, especially nitrogen oxides in combustion exhaust gases

ABSTRACT

A process for reducing pollutants, particularly nitrogen oxides from combustion gases during a combustion process that takes place while oxygen is supplied, includes providing oxygen needed for the combustion process by separating oxygen from a gas mixture containing oxygen and nitrogen in a two-step process including (a) enriching the gas mixture with oxygen in a first step to provide an enriched gas mixture; and (b) separating oxygen out of the enriched gas mixture in a second step, wherein, during at least one step oxygen depleted gas mixture is removed via an outlet provided with permeability means that cause the outlet to have a higher permeability for nitrogen than oxygen. A device for carrying out this process includes a device for separating oxygen out of a gas mixture containing oxygen and nitrogen including a housing having an inlet for the gas mixture; an outlet for the oxygen separated out of the gas mixture; and first and second apparatuses which have a respectively different permeability for oxygen and nitrogen, and which divide the device into first, second, and third chambers, wherein at least the second chamber is connected to an outlet provided with a permeability device that cause the outlet to have a higher permeability for nitrogen than for oxygen.

This Application is the National Stage of PCT/DE 96/00643, filed Apr. 6,1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for reducing pollutants, particularlynitrogen oxides, in combustion exhaust gases and a device for reducingpollutants.

2. Description of the Related Art

State of the Technology

Processes and devices for reducing pollutants in combustion exhaustgases in a combustion process that takes place with the supply of oxygenare known. For example, in internal combustion engines in motorvehicles, which use a fuel-air mixture that is ignited with the aid ofan ignition means, the nitrogen is removed from the oxygen-nitrogen-airmixture (atmosphere) prior to supply into the internal combustionengine, as disclosed in DE-PS 44 04 681. For this purpose, theoxygen-nitrogen-air mixture is guided across a barrier that isimpermeable to nitrogen. Consequently, combustion can continue with thesupply of atmospheric oxygen removed from the atmospheric air, while theatmospheric nitrogen present in the atmospheric air is not supplied tothe combustion process. The formation of nitrogen oxides during thecombustion process is prevented, or at least drastically reduced.

Moreover, ceramic components are known that have a membrane comprisingan oxygen ion-conducting material. Components of this type are used, forexample, as so-called lambda sensors to determine the oxygen content inexhaust gases of combustion processes. It is known that these oxygenion-conducting membranes have differing oxygen-conducting capabilitiesat different temperatures and under different pressure conditions.

So-called zeolites are further known from physical chemistry. They aredistinguished by a structure having large, internal hollow spaces thatare connected to one another by pores of defined size. These pores ofdefined size can be set in a range of a few tenths of an Angstrom by,for example, interspersed cations that move freely within the crystalgrid and can be exchanged in solution. If such zeolites are acted uponby an oxygen-nitrogen-air mixture, due to the steric effect only themolecules whose diameters are smaller than the width of the pore openingreach the interior of the crystal structure. Thus, a sieving effectoccurs. In the kinetic effect, certain molecules diffuse in and throughthe crystal structure faster than others, likewise causing a separatingeffect. If the oxygen-nitrogen-air mixture is fragmented, the separationof nitrogen and oxygen is based on the equilibrium effect. In thisinstance, different absorption forces are responsible for the strongerbonding of one component, for example nitrogen, than another component,such as oxygen.

SUMMARY OF THE INVENTION

The present invention provides a process of reducing pollutants,particularly nitrogen oxides, in combustion exhaust gases in acombustion process that takes place with the supply of oxygen, with theoxygen being removed from an oxygen-nitrogen-air mixture in a two-stepprocess, characterized in that, in at least one step, theoxygen-depleted oxygen-nitrogen-air mixture is carried off via an outletdevice provided with means that cause the outlet device to be morepermeable to nitrogen than oxygen.

The process of the invention, offers the advantage that relativelylittle energy can be used to remove the oxygen from theoxygen-nitrogen-air mixture. In accordance with the invention, afortification of oxygen is effected in the oxygen-nitrogen-air mixturein a first step (fortification step). In a subsequent, second step, theoxygen is removed, in pure or virtually pure form, from theoxygen-fortified oxygen-nitrogen-air mixture (removal or separationstep). A result of the oxygen fortification in the first step is a lowerenergy consumption for heating the oxygen-nitrogen-air mixture duringthe second step, because no unnecessarily large quantity of nitrogenneed be heated with the mixture. In addition, because of the oxygenfortification in the first step, the oxygen partial pressure of theoxygen-nitrogen-air mixture present in the second step can beapproximately doubled in comparison to the oxygen partial pressure inpure air. Therefore, the compression and/or heating energy to be used inthe second step can be reduced significantly. If the same quantity ofoxygen is to be removed, only a smaller quantity of theoxygen-nitrogen-air mixture must be compressed.

The fortification of the oxygen in the oxygen-nitrogen-air mixture thatis performed in the first process step can be achieved, for example,with the aid of an apparatus or barrier that is more permeable to oxygenthan nitrogen, such as a plastic membrane or a zeolite.

In the use of plastic membranes, in accordance with the invention anoxygen fortification of the oxygen-nitrogen-air mixture is performed ina first process step by means of a plastic membrane that is permeable todifferent degrees to oxygen and nitrogen. Oxygen can pass more quicklythrough the membrane, and is therefore fortified on the low-pressureside of the membrane.

If zeolites are used, they are exposed to the oxygen-nitrogen-airmixture at higher pressure, with the nitrogen preferably being absorbedand stored. This means that the air current passing through is fortifiedwith oxygen. The zeolite is purified of nitrogen by being exposed to alow gas pressure (regeneration step). In accordance with the invention,alternating operation between at least two zeolite stations is necessaryfor continuous operation. The gas currents are routed via flap valvessuch that one zeolite vessel is respectively undergoing theoxygen-fortification step, while the other is undergoing theregeneration step.

The air mixture passing through the barrier is thereforeoxygen-fortified, while an oxygen-depleted air mixture is carried offvia an outlet in the space in front of the barrier.

In a second process step, the oxygen-fortified oxygen-nitrogen-airmixture is supplied, for example, to a ceramic membrane for removal ofpure or virtually pure oxygen. The ceramic membrane preferably comprisesa mixed-conductive material, that is, a material that conducts bothoxygen ions and electrons. The ceramic membrane is preferably heated inaccordance with the invention. The invention may provide that only theceramic, not the oxygen-enriched oxygen-nitrogen-air mixture supplied toit, is heated. The supplied oxygen-fortified oxygen-nitrogen-air mixtureis, however, also advantageously heated, so it does not lead to coolingof the heated ceramic upon impact. In a particularly advantageousmanner, the supplied oxygen-fortified oxygen-nitrogen-air mixture can beacted upon by pressure. Pure or virtually pure oxygen is removed via themixed-conductive ceramic membrane, while an oxygen-depleted air mixtureis carried off via a second outlet in the space in front of the ceramicmembrane.

In a particularly advantageous manner, a further oxygen fortificationand thus an increase in the oxygen partial pressure can be achieved inthe first and/or second process step in that the respectiveoxygen-depleted air mixtures to be carried off are conducted acrossfurther apparatuses, for example plastic membranes, that have anincreased permeability for nitrogen and a reduced permeability foroxygen. The oxygen-nitrogen-air mixture that was carried off in thefirst process step has a smaller oxygen component than theoxygen-nitrogen-air mixture supplied to the first process step via theinlet, and that of the oxygen-nitrogen-air mixture supplied to thesecond process step; nevertheless, it contains oxygen, so the oxygenmigrates out undesirably. Likewise, in the second process step, anoxygen-depleted oxygen-nitrogen-air mixture is carried off, so anundesired out-migration of oxygen also occurs here. In accordance withthe invention, this out-migration of oxygen is prevented or reduced inthat the respective outlet means for the oxygen-depletedoxygen-nitrogen-air mixture contain further apparatuses that selectivelyhold back the oxygen; in other words, the means are more permeable tonitrogen than oxygen. This inhibits the undesired migration of oxygenout of the process, and the oxygen partial pressure acting as a drivingforce of fortification and/or separation at the membranes that are morepermeable to oxygen is maintained at a higher level than if thesefurther apparatuses were not provided. This significantly improves theenergy balance of the process of the invention.

The present invention additionally provides a device for reducingpollutants, particularly nitrogen oxides, in combustion exhaust gases ina combustion process that takes place with the supply of oxygen, thedevice having an arrangement for removing the oxygen from anoxygen-nitrogen-air mixture (atmosphere), with the device (10) havingtwo apparatuses (18, 18', 22) that are permeable to different degrees tooxygen O₂ and nitrogen N₂, the apparatuses being disposed between aninlet (14) for the oxygen-nitrogen-air mixture (16) and an outlet (26)for the removed oxygen O₂, characterized in that the device (10) issubdivided into three chambers (12, 20, 24) by the apparatuses (18, 18',22), and at least one chamber is connected to an outlet device (12, 20,44) that has a means (19, 21) that is more permeable to nitrogen thanoxygen.

The device of the invention for reducing pollutants, has the advantagethat a low-energy removal of oxygen from an oxygen-nitrogen-air mixtureis possible with simple means within a very small space. Because thearrangement has two apparatuses that are respectively permeable todifferent degrees to oxygen and nitrogen, the apparatuses being disposedbetween an inlet for the oxygen-nitrogen-air mixture and an outlet forthe removed oxygen, it is advantageously possible to use this device togenerate an oxygen-fortified oxygen-nitrogen-air mixture in a firstprocess step, from which mixture the oxygen is then removed in a secondprocess step. With the two apparatuses, the device can preferably besubdivided into three chambers, with the apparatuses being disposedbetween the individual chambers and permitting an independent treatmentof the oxygen-nitrogen-air mixture. Consequently, it is possible in asimple manner to achieve different oxygen concentrations in theindividual chambers, so the oxygen can be removed simply from a chamberhaving an increased oxygen concentration.

The invention particularly provides that the first and second chambersrespectively have an outlet for an oxygen-depleted oxygen-nitrogen-airmixture. In an especially advantageous manner, at least one, andpreferably both, of the outlets is or are provided with a furtherapparatus having a different permeability for oxygen and nitrogen,particularly being more permeable to nitrogen than oxygen. An apparatusof this type can be a membrane having increased permeability fornitrogen and a reduced permeability for oxygen, so the oxygen isselectively held back in the first and second chambers, and isadditionally fortified.

In a preferred embodiment of the invention, it is provided that thefirst apparatus operates pressure-dependently, and the second apparatusoperates pressure- and/or temperature-dependently. In a particularlypreferable embodiment of the invention, it can be provided that thefirst and second apparatuses can be acted upon by different pressuresindependently of one another, so the generation of additional pressuredrops can help to attain a further improvement in the energy balance ofthe oxygen fortification and removal. The further apparatuses preferablyprovided in the outlets likewise preferably operate pressure- and/ortemperature-dependently. Consequently, relatively simple, availablemedia, namely a pressure and a temperature, can be used to set thedevice at a different degree of oxygen removal.

Advantageous embodiments of the invention ensue from the other featuresdisclosed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail below by way of embodimentsillustrated in the attached drawings. Shown are in:

FIG. 1 schematically, an arrangement for removing oxygen from anoxygen-nitrogen-air mixture,

FIG. 2 schematically, a further arrangement for removing oxygen from anoxygen-nitrogen-air mixture, in which the outlets are provided withfurther apparatuses,

FIG. 3 schematically, a further arrangement for removing oxygen from anoxygen-nitrogen-air mixture, in which a chamber is subdivided into alow-pressure region and a high-pressure region, and

FIG. 4 a schematic arrangement of an apparatus for fortifying the oxygenin an oxygen-nitrogen-air mixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a device for removing oxygen from anoxygen-nitrogen-air mixture 16, the device being generally indicated by10. The device 10 has a first chamber 12, which is provided with aninlet 14 for the oxygen-nitrogen-air mixture 16. The first chamber 12 isseparated from a second chamber 20 by a first apparatus 18, which willbe explained below. The second chamber 20 is separated from a thirdchamber 24 by a second apparatus 22--also to be explained below. Thethird chamber 24 has an outlet 26 for oxygen 28 that has been removed bythe device 10. The first chamber 12 is further provided with an outlet30 for the oxygen-nitrogen-air mixture 16; a throttle valve 32 isdisposed in the outlet 30. Means 34 for exerting a first pressure p1 onthe first chamber 12 are further provided at the first chamber 12. Thesecond chamber 20 includes means 36 for exerting a pressure p2 on thesecond chamber 20. Furthermore, a heating device 38 is provided in thesecond chamber 20; this device has, for example, a heating spiral 40that is disposed inside the chamber 20 and can be connected to a heatingvoltage by way of connections 42. Moreover, the second chamber 20 has anoutlet 44, in which a throttle valve 46 is disposed. The third chamber24 has an inlet 48, in which a further throttle valve 50 is disposed.

The first apparatus 18 comprises a membrane 52, which has a differentdegree of permeability (permeation rate) for oxygen O₂ and nitrogen N₂when a pressure difference exists between the first chamber 12 and thesecond chamber 20. The second apparatus 22 comprises a mixed-conductiveceramic membrane 54, a so-called perovskite.

With the device of the invention, which is illustrated in FIG. 1, theprocess of the invention progresses as follows:

The first chamber 12 is acted upon by an oxygen-nitrogen-air mixture 16by way of the inlet 14. This oxygen-nitrogen-air mixture typicallycomprises the atmospheric air of the device 10. The throttle valve 32 ofthe outlet 30 of the first chamber 12 effects a smaller cross section ofthe outlet 30 with respect to the cross section of the inlet 14. Thefirst chamber 12 is acted upon by the pressure p1 by way of the means34. This establishes a pressure difference between the chamber 12 andthe second chamber 20 that is present on both sides of the membrane 52.Because the membrane 52 is permeable to different degrees to oxygen O₂and nitrogen N₂ when a pressure difference exists, the oxygen O₂ candiffuse more quickly through the membrane 52 than the nitrogen N₂. As aresult, a larger proportion of oxygen O₂ than nitrogen N₂ diffuses intothe second chamber 20. The first chamber 12 is preferably permanentlyacted upon by the pressure p1, so oxygen O₂ can continuously diffusethrough the membrane 52 in larger quantities, and nitrogen N₂ cancontinuously diffuse through in smaller quantities. The surplusoxygen-nitrogen-air mixture 16 is continuously discharged via thethrottle valve 32 of the outlet 30 in a controlled manner; the exitingoxygen-nitrogen-air mixture 16 has a smaller oxygen O₂ component thanthe oxygen-nitrogen-air mixture supplied via the inlet 14.

Thus, with the first apparatus 18, an oxygen-nitrogen-air mixture ispresent in the second chamber 20 that has a larger oxygen O₂ componentthan the oxygen-nitrogen-air mixture 16 present at the inlet 14. Thismixture fortified with oxygen O₂ is now heated by the heating device 38and acted upon by a pressure p2 exerted by the means 36. This compressesthe oxygen-nitrogen-air mixture in the second chamber 20, and presses itagainst the ceramic membrane 54. A controlled pressure buildup in thesecond chamber 20 can be established by a setting of the throttle valve46. The ceramic membrane 54 is mixed-conductive, effecting anacceleration of the oxygen ions in the direction of the third chamber24. Because the oxygen ions have a negative potential, an opposingelectron conduction takes place through the ceramic membrane 54. Thismembrane is thus configured as a mixed-conductive ceramic membrane 54.The gradient of the oxygen partial pressure across the ceramic membrane54 functions as the driving force of the oxygen-ion transport. Hence,the proportion of oxygen ions 20⁻² can be established through thesetting of a pressure p2 in the second chamber 20; the proportiondiffuses from the second chamber 20 into the third chamber 24 during aspecific period of time.

The heating device 38 is provided because the oxygen-nitrogen-airmixture in the chamber 20 must have a certain thermal potential for thediffusion process of the oxygen ions that is to take place through theceramic membrane 54. Here, however, a considerably smaller quantity ofenergy is necessary than in conventional heating of theoxygen-nitrogen-air mixture, because an oxygen-nitrogen-air mixture thatis fortified with oxygen O₂ must be heated. The relative proportion ofnitrogen N₂ is therefore smaller with respect to the total composition,so relatively little nitrogen must be heated with the mixture.Furthermore, a higher oxygen partial pressure results inside the secondchamber 20 due to the oxygen-fortified oxygen-nitrogen-air mixture, sothe pressure p2 to be exerted for the diffusion of oxygen ions throughthe ceramic membrane 54 can be relatively low. As a result, heating andcompression energy are saved.

Overall, the coupling, that is, the arrangement of the membrane 52(oxygen fortification) before the ceramic membrane 54 (oxygenseparation), results in a lower specific separation output (totaloutput/separated quantity of oxygen) because of the smaller quantity ofnitrogen N₂ to be heated and compressed, and because of the increasedoxygen partial pressure as the driving separation force, due to theaforementioned oxygen concentration.

The oxygen ions that have diffused into the third chamber 24 can bemixed with an inert gas via the inlet 48, which gas takes over thetransport of the oxygen O₂ via the outlet 26 to a combustion chamber inthat a combustion process takes place with the supply of oxygen. Acombustion exhaust gas from the combustion process, for example, can beused as an inert gas for transporting the oxygen O₂ ; such a gas isre-supplied to the combustion process via a short-circuit line. In thisinstance sufficient oxygen from the diffused-in oxygen ions is availablefor the combustion process. The nitrogen N₂ originally present in theoxygen-nitrogen-air mixture 16 is therefore excluded from involvement inthe combustion process, so the formation of nitrogen oxides NO_(x)during the combustion process is drastically reduced, and only aresidual component remains due to the nitrogen component of the fuel.

The arrangement of FIG. 2 corresponds to that of FIG. 1, except that theoutlets 30 and 44 are provided with further apparatuses 19 and 21. Theseapparatuses respectively comprise a membrane that has an increasedpermeability for nitrogen and a reduced permeability for oxygen. In FIG.2, the apparatus 19 is on the pressure side of the throttle valve 32,and the apparatus 21 faces away from the pressure of the throttle valve46. Other arrangements of the throttle valve 32 or 46 relative to theapparatus 19 or 21 are also possible, however; for example, theapparatus 19 can be disposed on the side of the throttle valve 32 facingaway from the pressure, and/or the apparatus 21 can be disposed on theside of the throttle valve 46 facing the pressure. In accordance withthe invention, it can also be provided that the throttle valve 32 and/or46 is or are replaced by the apparatus 19 and/or 21.

The process executed with this device corresponds to the one explainedin conjunction with FIG. 1, except that the oxygen-depletedoxygen-nitrogen-air mixture exiting the outlets 30 and 44 is conductedthrough the further apparatuses 19 and 21 that are more permeable tonitrogen than oxygen, inhibiting the undesired migration of oxygen outof the chambers 12 and 20. A result of this is an oxygen partialpressure in these two chambers that is maintained at a higher level incomparison to a device that is not equipped with these furtherapparatuses 19 and/or 21, so the energy to be used for fortification andremoval is further reduced.

FIG. 3 schematically shows a device for fortification and removal ofoxygen, the device being indicated by 10' and corresponding to thedevice shown in FIG. 2, except that the chamber 20 is subdivided into alow-pressure region 20' that is associated with the apparatus 18 and ahigh-pressure region 20" that is associated with the apparatus 54. Thelow-pressure region 20' and the high-pressure region 20" are coupledwith respect to pressure by the apparatus 36, which can be configured asa vacuum pump, for example. The apparatus 18 can be acted upon by apressure other than the one acting on the apparatus 54. The subdivisionof the chamber 20 into regions 20' and 20" with different pressureconditions permits additional pressure drops that drive the oxygenfortification and separation to be effected. Of course, the inventionalso encompasses a multiple-stage embodiment of the apparatus 36, so thesetting of essentially independent pressures is permitted, for example,in regions 20' and 20". The low-pressure region 20' advantageously hasan underpressure relative to the chamber 12, so a pressure drop thatadditionally drives the oxygen fortification is established. Conversely,the high-pressure region 20" has an overpressure relative to the chamber24, so an additional pressure drop that is favorable for the oxygenseparation is present here.

FIG. 4 shows a further embodiment of the first apparatus 18. Theapparatus 18' of FIG. 4 can be used in place of the apparatus 18explained in conjunction with FIG. 1, in which case, for betterunderstanding, identical parts having identical functions are providedwith the same reference numerals, although they have different designs.

The apparatus 18' includes the inlet 14 for the oxygen-nitrogen-airmixture 16. The inlet 14 is connected to a first channel 56 and a secondchannel 58. The channels 56 and 58 can be alternatingly connected to theinlet 14, or separated from it, by way of an apparatus 60, for example aflap. A region 62 that is equipped with a zeolite 64 is provided in thechannel 56. Correspondingly, the channel 58 includes a region 66 that isalso equipped with a zeolite 64. In the regions 62 and 66 of thechannels 56 and 58 having the zeolite 64, the zeolite 64 advantageouslyextends over the entire cross section of the channels 56 and 58. Thechannels 56 and 58 terminate in the second chamber 20 of the device 10(FIG. 1). Between the regions 62 and 66 having the zeolite 64 and thechamber 20, the channels 56 and 58 are connected via a branch 68 and68', respectively, to a conveying device 70, for example a pump. Betweenthe branches 68 and 68' and the second chamber 20, the channels 56 or 58can respectively be connected to or separated from the chamber 20 by wayof a blocking device 72 that can be alternatingly actuated. The blockingdevice 72 has two flap valves 74 and 74', which are coupled to oneanother and alternatingly connect the conveying device 70 or the chamber20 to the channel 56 or 58.

The apparatus 18' shown in FIG. 4 functions as follows:

In the initial state, the apparatus 60 closes the channel 56, so thechannel 58 is connected to the inlet 14. At the same time, the blockingdevice is switched such that the flap valve 74' seals the branch 68' andthe channel 58 is connected to the second chamber 20. The first channel56 is connected to the conveying device 70 by way of the branch 68,while the flap valve 74 separates the channel 56 from the second chamber20. Via the inlet 14, the apparatus 18' is acted upon by theoxygen-nitrogen-air mixture 16 at a pressure of approximately 1 bar. Theoxygen-nitrogen-air mixture 16 is thus conducted to the zeolite 64disposed in the region 66 of the channel 58. The zeolite 64 possesses astructure that allows the nitrogen molecules of the oxygen-nitrogen-airmixture 16 to be absorbed, while the oxygen molecules can pass throughthe region 66. Thus, an oxygen-nitrogen-air mixture 16 is present in thesecond chamber 20 that has a higher oxygen proportion than at the inlet14. As already explained in conjunction with FIG. 1, the oxygen is thenseparated out of this oxygen-nitrogen-air mixture fortified with oxygen.

Because the region 66 having the zeolite 64 is known to have only acertain storage capacity, and therefore the nitrogen absorption can leadto saturation, the apparatus 18' can be selectively reversed, forexample with time control, as follows. The flap of the apparatus 60 isreversed such that the channel 56 is connected to the inlet 14, whilethe channel 58 is separated from the inlet 14. At the same time, theblocking device 72 is reversed, so the flap valve 74 blocks the branch68 and connects the channel 56 to the second chamber 20. The flap valve74' simultaneously releases the branch 68' and separates the channel 58from the second chamber 20. Now the zeolite 64 in the region 66 is actedupon by an underpressure by way of the conveying device 70. This isknown to effect a regeneration of the zeolite 64 in the region 66. Thischange in pressure at the zeolite 64 in the region 66 causes thenitrogen that has previously been absorbed from the oxygen-nitrogen-airmixture 16 to be sucked up by the conveying device 70, so the mixture ispartially purified of nitrogen molecules.

While the zeolite 64 in the region 66 is regenerated, theoxygen-nitrogen-air mixture 16 is conducted via the zeolite 64 in theregion 62 of the channel 56. Here the oxygen-nitrogen-air mixture 16fortified with oxygen is supplied in the above-described manner to thesecond chamber 20 via the channel 56.

The selected design of the apparatus 18', particularly the arrangementof the apparatus 60 or the blocking device 72, assures a continuousoperation, because the zeolite 64 in the regions 62 and 66 alternatinglyabsorbs and regenerates the nitrogen of the oxygen-nitrogen-air mixture.

It is possible to fortify the oxygen in the oxygen-nitrogen-air mixtureby over 50% with the apparatus 18'. As already explained in conjunctionwith FIG. 1, this oxygen-fortified oxygen-nitrogen-air mixture is nowsupplied to the second apparatus 22; that is, it is guided to themixed-conductive ceramic membrane 54 with the effect of temperatureand/or pressure.

A very advantageous application of the device 10 ensues, for example,from the supply of a fuel-air mixture for an internal combustion enginein motor vehicles. The release of nitrogen oxides is prevented, or atleast significantly reduced, in motor vehicles equipped with the device10 of the invention.

What is claimed is:
 1. A process for reducing pollutants, particularlynitrogen oxides from combustion gases during a combustion process thattakes place while oxygen is supplied, the process comprising:providingoxygen needed for the combustion process by separating oxygen from a gasmixture containing oxygen and nitrogen in a two step process comprisedof:a. enriching the gas mixture with oxygen in a first step to providean enriched gas mixture; and b. separating oxygen out of the enrichedgas mixture in a second step, wherein, during at least one step, oxygendepleted gas mixture is removed via an outlet provided with permeabilitymeans that cause the outlet to have a higher permeability for nitrogenthan oxygen.
 2. The process according to claim 1, wherein enriching thegas mixture with oxygen in the first step is accomplished by one ofabsorption of nitrogen or by means of a membrane which has a higherpermeation rate for oxygen than for nitrogen.
 3. The process accordingto claim 1, wherein separating oxygen out of the enriched gas mixture inthe second step is accomplished with an oxygen-ion conducting membrane.4. The process according to claim 3, wherein the gas mixture is suppliedin at least one step of the two step process under pressure to at leastone of (a) the oxygen-ion conducting membrane and (b) the outlet.
 5. Theprocess according to claim 3, wherein the oxygen separated out in thesecond step is diffused into the combustion gases by means of theoxygen-ion conducting membrane, and wherein diffusion of the oxygen intothe combustion gases per unit of time is influenced by adjusting atleast one of pressure of the enriched gas mixture and temperature of theenriched gas mixture.
 6. The process according to claim 1, furthercomprising compressing the enriched gas mixture from the first step. 7.The process according to claim 1, further comprising heating theenriched gas mixture from the first step.
 8. A device for carrying outthe process according to claim 1, comprising:a device for separatingoxygen out of a gas mixture containing oxygen and nitrogen comprised ofa housing having an inlet for the gas mixture; an outlet for the oxygenseparated out of the gas mixture; and first and second apparatuses whichhave a respectively different permeability for oxygen and nitrogen, andwhich divide the device into first, second, and third chambers, whereinat least the second chamber is connected to an outlet provided withpermeability means that cause the outlet to have a higher permeabilityfor nitrogen than for oxygen.
 9. The device according to claim 8,wherein the first and second apparatuses are arranged independent ofeach other.
 10. The device according to claim 8, wherein the firstapparatus is pressure dependent in operation.
 11. The device accordingto claim 8, wherein the second apparatus is at least one of pressuredependent and temperature dependent in operation.
 12. The deviceaccording to claim 8, wherein first chamber is connected to the inletand the third chamber is connected to the outlet for the oxygenseparated out of the gas mixture.
 13. The device according to claim 8,wherein the first apparatus is arranged between the first chamber andthe second chamber, and wherein the second apparatus is arranged betweenthe second chamber and the third chamber.
 14. The device according toclaim 8, wherein first and second chamber may be pressurized separatelywith respective pressures p1 and p2.
 15. The device according to claim8, wherein the second chamber is provided with heating means.
 16. Thedevice according to claim 8, wherein the first apparatus contains atleast in some regions a zeolite having a nitrogen-absorbing structure.17. The device according to claim 16, wherein the apparatus containschannels having respective regions containing zeolite which may bealternately filled with the gas mixture and connected to the secondchamber.
 18. The device according to claim 16, further comprising aconveying device associated with the first apparatus for admitting thezeolite under a low pressure for regeneration thereof.
 19. The deviceaccording to claim 8, wherein the second apparatus comprises amixed-conductive ceramic membrane which is arranged between the secondchamber and the third chamber.